This invention relates to ion probe microanalysis, and more particularly, to the collection and transport of ions resulting from the impact of an ion beam on a target surface.
Ion probe microanalysis is a relatively new technique for characterizing solid material samples with high spatial resolution. A number of ion probe analyzers are presently available which operate by bombarding a sample under investigation with energetic heavy ions. This causes secondary atoms and atom clusters, a portion of which are ionized, to be sputtered from the sample. These secondary ions are then analyzed by means of a mass spectrometer. To obtain the necessary ions for analysis, a secondary ion collection system is typically placed between the final lens of the ion beam source and the target surface. The collected secondary ions are then transported to a spectrometer for SIMS (secondary ion mass spectrometry) analysis. The various applications for ion probe microanalyzers include surveying semiconductor doping profiles, failure analysis for electronic devices, characterization of rock samples, analysis of small airborne particles in environmental research, biochemistry and document authentication.
In conventional ion microprobes, the secondary ion collection system is relatively large, and its placement between the final lens of the ion beam source and the target surface requires the final lens to be spaced a substantial distance from the target. This in turn requires the lens to have a long working distance, resulting in lens aberrations which are too large to produce an effective beam probe of submicrometer size. A summary of various available ion probe microanalyzers, including their secondary ion collection and transport systems, is presented in a review article by H. Liebl, "Ion Probe Microanalysis", Journal of Physics E. 16, pages 797-808 (U.K., 1975). These systems are designed to have relatively large working spaces, with an accompanying degradation in the size of the probe beam.
While ion microprobes with a shorter working distance and correspondingly smaller and more accurate beam size can be constructed, the necessary reduction in working space between the final lens and the target surface would rule out the use of most presently available secondary ion collection and transport systems. Existing systems generally use either large magnetic lenses or einzel lenses for secondary ion collection and transport. The magnetic lens systems comprise iron pole pieces with associated windings to provide magnetic fields for moving the secondary ions. Since it is somewhat difficult to transport ions with magnetic fields, relatively large magnetic lens structures are required.
The prior art systems generally use magnetic prism mass analyzers to analyze the collected secondary ions. While these analyzers are suitable for the relatively large scale secondary ion collection and transport systems in use heretofore, they generally occupy too large a space to be compatible with much more compact and higher resolution microprobes. The Liebl review article recognizes the limitations imposed by the large working distances required by conventional secondary ion extraction, and proposes that the secondary ions be extracted backwards through the objective lens (page 805, FIGS. 23.gtoreq.25). The proposed system, however, requires the use of multiple reflecting surfaces for the secondary ions, and may not impart sufficient energy to the ions to completely extract them from the system for analysis.